The invention relates to windings for electrical machines such as motors and generators and, more particularly, to a .DELTA.-connected, two-layer, three-phase winding for electrical machines, especially superconducting machines, with at least two shunt connected sets of coils per phase.
In the design of electrical machines, the rated voltage of the machine and the total volume of the insulation have a great influence on the economy of the machine. It is typically the aim to attain a maximum value for rated voltage of the machine while keeping the volume of the insulation at a minimum value.
In one standard design two-layer, three-phase winding used in large electro-unit engineering and referred to hereinafter as a conventional winding, the conductor rods are arranged in slots of the stator plate in two radially superposed layers. The three phase windings are either .gamma.-connected or .DELTA.-connected (delta-connected) and the sets of coils, of which there are at least two for each phase, are connected either in parallel (shunt) or in series.
With windings of this type a relatively high voltage will occur at the so-called phase change, between the conductor rods and the plate iron, between conductor rods belonging to the same or a different phase, either superposed or located side-by-side, and between adjacent conductors of one layer of the coil end. For example, in case of a .DELTA.-connection with two sets of shunt connected coils per phase, the voltage at the phase changes, e.g. the voltage between adjacent winding elements carrying different phases of the three-phase input voltage, will be the full phase-to-phase voltage. The statements made above are substantially also true in case of air-gap windings where the conductor rods are placed within an insulating cylinder. Therefore the conductors need to be insulated against the high voltages involved, a fact which is especially disadvantageous in case of conventional machines of ultra-large size as well as superconducting machines because their spatial and insulation requirements will go beyond their permissible limits due to the high voltages arising therein.
In another known winding arrangement, described in U.S. Pat. No. 3,743,875, which is particularly suitable for superconducting machines but can also be used in connection with conventional machines, the insulation volume is reduced by specific arrangements involving construction as well as electrical connections. The stator of this electrical machine contains a central active component of smaller external diameter in which the conductor rods extend in the axial direction, forming two superposed layers. Each end of the active component carries one section of greater external diameter, and the end sections of the conductor rods are spread forming four radially superposed layers. The end sections of the conductor rods of each layer are bent helically by 90 electrical degrees with the bends of superposed layers oriented in such manner that they will cross each other to allow the necessary series connections.
The end sections of the conductor rods located side-by-side are insulated from each other against a voltage amounting to four winding voltages wherein a winding voltage is the voltage across a single conductor rod of a winding (i.e. from one end turn of a conductor rod to the opposite end turn). Furthermore, there are arranged with in the two end sections of the stator, between all four superposed layers, insulating cylinders which must be able to withstand a substantially greater voltage because there occurs a phase-meshed voltage across the intersecting conductors. In general, this arrangement is complicated, is costly and requires a great amount of material.
Published German application 2 518 786 shows a six-phase winding which consists of two three-phase sub-windings, displaced by 30 electrical degrees relative to each other, their phase zones each taking up 30.degree. circumferentially. The two sub-windings consisting of standard coils without cross-connections are wound in opposite directions and are installed in the stator with terminals located at diverse sides. This arrangements limits the voltage at the phase changes to 52% of the phase voltage of the equivalent three-phase winding, produced by a series connection of the two sub-windings. This solution has the great disadvantage that it requires winding connections at both sides of the stator, with six leads on each side, as well as an additional transformer winding.
U.S. Pat. No. 2,745,029 shows another solution for the purpose of reducing the potential to ground and at the phase change of a three-phase winding. The centers of at least one parallel branch in each of the three phases are tapped, connected to each other and grounded. The parallel branches, built up from standard coils without cross-connections are wound in opposite directions and are installed in the stator with terminals located at diverse sides. The voltage at the phase changes amounts in this case to 27% of the phase voltage. However, the number of winding leads--nine at one side, and six at the other side of the stator--exceeds even the number of leads needed by the above-described arrangement of the German application. These two solutions, in fact, have one disadvantage in common, namely the need for three additional high-voltage lead-throughs at the generator transformer.
It is a primary object of this invention to provide a three-phase winding of the above-described type where the load voltage of the conductor rods within one layer in the active portion as well as in the coil end of an electrical machine is reduced by means of inexpensive layouts in design, so that a high machine voltage will be possible while the insulation volume is held to a low value, in order to save space as well as cost.
A three-phase winding to solve this problem is characterized by the features that in the case of two adjacent sets of coils the coils of one set are cross-connected and the coils of the other set are non-crossed, whereby the coil end overhangs of the crossed sets are longer than the overhangs of the non-crossed sets, and where the difference in voltage between adjacent conductor rods of one layer amounts at all phase shifts at the most to one winding voltage during operations.
This feature is accomplished because the adjacent conductors are not removed from their common terminal at the most by one turn each at the phase change and because, due to the phase displacement of 60.degree. within the two volt turns, their difference, that is the voltage across the adjacent conductor rods involved, will likewise amount to one turn voltage only.
The three-phase winding proposed by the invention, can be a two-pole or a multi-pole, a chorded or an unchorded winding. It makes possible a saving of space as well as of insulation, and is particularly suitable for use in connection with conventional ultra-large machines and superconducting machines.
The foregoing and other objects and advantages of the present invention will become apparent to one skilled in the art to which the invention pertains from the following detailed description when read in conjunction with the appended drawings.